Semiconductor piezoresistor

ABSTRACT

A piezoresistor having a base substrate with a quantum well structure formed on the base substrate. The quantum well structure includes at least one quantum well layer bounded by barrier layers. The barrier layers are formed from a material having a larger bandgap than the at least one quantum well layer.

RELATED APPLICATION

[0001] This application is a continuation of application Ser. No.09/500,408, filed Feb. 8, 2000. The entire teachings of the aboveapplication are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] Strain gages are commonly used to detect stresses in materials,changes in pressure and temperature, etc. Typically, strain gages employone or more piezoresistive elements or piezoresistors which experience achange in resistance when subjected to strain induced by physical and/orchemical stimuli. The piezoresistive elements found in a conventionalstrain gage are usually formed of several loops of fine wire or aspecial foil composition. In use, the gage is bonded to the surface ofthe object to be analyzed. When the object is deformed in response toparticular stimuli, the piezoresistive elements of the gage are strainedwhich alters the resistance of the piezoresistive elements. The changein resistance is measured and then is correlated to the level of strainexperienced by the object.

[0003] Recently, micro-electromechanical sensors have been developedthat are manufactured by semiconductor microelectronic processing andprecision etching technologies. These sensors can be employed formeasuring parameters such as pressure, acoustic vibrations, inertia(acceleration, vibration, shock), gas concentration, temperature etc.Such sensors typically employ micromechanical elements (membranes,cantilever beams, microbridges, tethered proof masses, etc.) which areperturbed by physical and/or chemical stimuli, with the magnitude of theperturbation being related to the magnitude of the physical or chemicalstimuli. Typically, piezoresistors are positioned on the micromechanicalelement at high-stress locations of the micromechanical element (forexample, at the edge of a membrane). The sensitivity of such sensors isproportional to the piezoresistive gage factor of the piezoresistors,defined as: GF=ΔR/Rε, the relative change in resistance ΔR/R with strainε.

[0004] Silicon is a common material for forming the piezoresistors inmicro-electromechanical sensors and has a gage factor that is suitablefor various applications. However, in some instances, a higher gagefactor is desirable so that the sensitivity of the sensor incorporatingthe piezoresistor can be increased, or alternatively, themicromechanical element on which the piezoresistor is positioned, can bestiffened for increased mechanical strength without reducing thesensitivity of the sensor. Attempts have been made to producepiezoresistors with higher gage factors than silicon, however, suchattempts have not produced significantly higher gage factors withconsistant piezoresistive properties.

SUMMARY OF THE INVENTION

[0005] The present invention is directed to a piezoresistor having agage factor that is significantly higher than current piezoresistivedevices. The piezoresistor of the present invention includes a basesubstrate with a quantum well structure formed on the base substrate.The quantum well structure has at least one quantum well layer bound orsandwiched by barrier layers. The barrier layers are formed from amaterial having a larger bandgap than the at least one quantum welllayer.

[0006] In preferred embodiments, each quantum well layer in the quantumwell structure is less than about 1000 Å thick and is more preferablyabout 5 Å to 30 Å thick. The barrier layers in the quantum wellstructure are less than about 1000 Å thick and are more preferably about5 Å to 50 Å thick. The quantum well structure may have one or morequantum well layers with about 5 to 10 layers being preferred. Selectedlayers in the quantum well structure may be doped. The base substrate ispreferably a single crystal and the layers of the quantum well structureare formed by epitaxial growth.

[0007] In one embodiment, the base substrate is a wafer, thin film orthin foil. In another embodiment, the base substrate is a micromachinedmechanical element of a sensor on which the quantum well structure isfabricated.

[0008] The present invention piezoresistor can be fabricated fromsemiconductor materials which are resistant to harsh conditions, suchas, high temperatures, high pressure, and corrosive or reactive gasenvironments. Such semiconductor materials are typically compatible withmicroelectronic devices and can be manufactured in batches. This makesthe present invention piezoresistor low cost in comparison to metalstrain gages. In addition, the piezoresistor can be fabricated either asa strain gage that is bonded to an object or structure to be analyzed,or as part of an electromechanical sensor for measuring parameters, suchas, pressure, acceleration, vibration etc.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0010]FIG. 1 is a schematic cross-sectional view of a preferredpiezoresistor of the present invention.

[0011]FIG. 2 is a band structure diagram for the quantum well structureof the piezoresistor of FIG. 1. Energy of the first quantized minibandE₁ changes with quantum well thickness a, which in turn depends on thestress applied. Also indicated are the bandgap energies E_(g) (A), E_(g)(B) and the barrier heights in the conduction and valence bandsE_(B)(CB), E_(B)(VB), with A and B being the barrier and quantum welllayers, respectively.

[0012]FIG. 3 is a schematic drawing of current I flowing through aquantum well.

[0013]FIG. 4 is a schematic side view of a piezoresistor of the presentinvention bonded to the surface of a contoured object.

[0014]FIG. 5 is a plan view of a portion of an electromechanical sensorshowing a multiple quantum well structure formed on a mechanical elementof the sensor.

[0015]FIG. 6 is a schematic view of a preferred method for formingelectrical contacts on a piezoresistor of the present invention. Ohmniccontacts are fabricated on the sidewalls, to measure lateral electricaltransport along the quantum wells.

DETAILED DESCRIPTION OF THE INVENTION

[0016] A description of a preferred embodiment of the invention follows.Referring to FIG. 1, piezoresistor 10 includes a single crystal basesubstrate 14, a thin insulating layer 16 deposited on the base substrate14, and a quantum well semiconductor structure 12 deposited over theinsulating layer 16. Typically, base substrate 14 is a mechanicalelement 26 of an electromechanical sensor 24 (FIG. 5), a thin bulk waferor a thin film or foil. When base substrate 14 is a wafer, film or foil,base substrate 14 must be thin enough so that piezoresistor 10 can bebonded to an object and will deform with the object when the object issubjected to stress. Insulating layer 16 electrically isolates thequantum well structure 12 from base substrate 14. Quantum well structure12 consists of thin film smaller bandgap quantum well semiconductorlayers 18 and larger bandgap barrier layers 20. The barrier layers 20maybe a semiconductor or an insulator. Typically, each smaller band gaplayer 18 is bounded or sandwiched between two larger bandgap layers 20which achieves quantum confinement of carriers within the smallerbandgap layer 18. The bottom layer 18 may be on the insulating layer 16as shown in FIG. 1, where the insulating layer 16 serves as a barrierlayer. The quantum well layers 18 are preferably doped n-type butalternatively, may be doped p-type or undoped. In addition, the barrierlayers 20 can be doped instead of the quantum well layers 18.

[0017] Referring to FIG. 2, quantum confinement of carriers is achievedin the quantum well layers 18 with electrons at a first quantizationminiband E₁. Miniband E₁ is shown in the conduction band of the quantumwell, which corresponds to n-type doping. Alternatively, p-type dopingwould result in miniband E₁ being in the valence band. It is understoodthat more than one miniband E₁, E₂, . . . can exist in the quantum well.FIG. 3 depicts the manner in which current I flows in a quantum welllayer 18. The barrier layers 20 have a barrier height E_(B)(CB) that ishigher than the first quantization miniband E₁. The miniband energy E₁depends on the quantum well thickness as follows: $\begin{matrix}{E_{1} = \frac{h^{2}}{8m^{*}a^{2}}} & \left( {{Eq}.\quad 1} \right)\end{matrix}$

[0018] where:

[0019] h is Planck's constant,

[0020] m* is the effective mass of the majority carrier in the quantumwell layer 18, and

[0021] a is the thickness of the quantum well layer 18.

[0022] Quantum well structure 12 has a resistance, which is the parallelcombination of the resistances of each individual quantum well layer 18.The lateral resistance R of an individual quantum well layer 18 dependson the number of carriers excited from the Fermi level to E₁ and thusdepends on E₁ as follows: $\begin{matrix}{R \propto e^{\frac{({E_{1} - E_{F}})}{kT}}} & \left( {{Eq}.\quad 2} \right)\end{matrix}$

[0023] where:

[0024] E_(F) is the Fermi level

[0025] k is Boltzmann's constant; and

[0026] T is temperature.

[0027] The band structure and thus the carrier transport is highlysensitive to the stresses which modulate the quantum well thicknessdimension a. For example, the application of a compressive stressreduces the quantum well dimension a, and in turn, increases (E₁-E_(F)),thereby resulting in a large increase in resistance R due to theexponential dependence of R on (E₁-E_(F)).

[0028] Substituting Eq. 1 for E₁ in Eq. 2 and assuming E_(F) to beconstant results in: $\begin{matrix}{R = {Ce}^{\frac{h^{2}}{8m^{*}a^{2}{kT}}}} & \left( {{Eq}.\quad 3} \right)\end{matrix}$

[0029] where:

[0030] C is a constant.

[0031] The relative change in resistance R at a particular temperature,can be found as a function of the quantum well dimension a bydifferentiating Eq. 3: $\begin{matrix}{\frac{dR}{R} = {\frac{{- 2}h^{2}}{8m^{*}{kT}}\frac{da}{a^{3}}}} & \left( {{Eq}.\quad 4} \right)\end{matrix}$

[0032] where:

[0033] da/a is the strain ε in the quantum well structure 12.

[0034] The gage factor, defined as (AR/Rε), is inversely proportional toa². High sensitivity is achieved with piezoresistors having large gagefactors. Highest sensitivity is achieved by using very thin quantum welllayers 18 as follows from the inverse a² dependence. Although FIG. 1depicts multiple layers in the quantum well structure 12, a singlequantum well layer is sufficient to realize the change in resistance asa result of strain in the film. However, large numbers (5 to 10) ofquantum well layers are preferred to enable easier measurement. In FIG.2, the band structure shows a Type I superlattice structure, whereconfinement of both electrons (in the conduction band) and holes (in thevalence band) is possible. However, the electrons, having smallereffective mass m*, enable higher sensitivity. Carrier confinement canalso be achieved using Type II superlattice structures, where only onetype of carrier (electrons or holes) is confined to a layer and isactive in electrical conduction.

[0035] In use, when analyzing a structure or object, for example, anaircraft wing, engine components, etc., a piezoresistor 10 having awafer, film or foil base substrate 14 is employed. The base substrate 14is bonded to the structure of object 22 (FIG. 4). When the surface ofthe structure of object 22 has contours, a film or foil base substrate14 is preferably employed because such a base substrate 14 allowspiezoresistor 10 to conform to the contours. Once the structure orobject 22 is subjected to stress, the piezoresistor 10 experiencesstrain. The strain changes the dimensions of the quantum well layer orlayers 18, which thereby changes the resistance of the quantum wellstructure 12. The quantum well structure 12 is electrically connected toa measurement circuit. The change in resistance is measured by themeasurement circuit, for example using a Wheatstone-bridge circuit. Thischange in resistance is then correlated to the strain of the underlyingstructure or object 22.

[0036] When base substrate 14 is a mechanical component or element of anelectromechanical sensor 24, for example, a cantilevered beam 26 (FIG.5), or a membrane, microbridge, tethered proof mass etc., the multiplequantum well structure 12 is formed on the mechanical element in a tracepattern with electrical contacts 28 on the two ends of the trace. Thequantum well structure 12 is deposited on the mechanical element 26 inthe desired pattern by employing standard masking techniques. Byconnecting the quantum well structure 12 to a measurement circuit, forexample, a Wheatsone-bridge circuit, the change in resistance of thequantum well structure 12 can be determined when the mechanical element26 of the electromechanical sensor 24 is perturbed by external stimuli.

[0037] A more detailed description of piezoresistor 10 now follows. Basesubstrate 14 is preferably a single crystal because a single crystalprovides a high quality surface that ensures that the overlying layers16/18/20 will be of uniform thickness and have a high degree ofcrystalline perfection. Typical materials for base substrate 14 aresilicon (Si), gallium arsenide (GaAs), silicon carbide (SiC) orsapphire, however, other suitable single crystal materials or amorphousor polycrystalline materials can be employed.

[0038] Insulating layer 16 is preferably about 1000 Å to 10,000 Å thickwith about 5000 Å being preferred. A preferred material for insulatinglayer 16 is aluminum nitride (AlN). Alternatively, other suitableresistive materials can be employed. However, when base substrate 14 ismade of an insulating material, insulating layer 16 can be omitted.

[0039] The larger bandgap barrier layers 20 of quantum well structure 12are preferably about 5 Å to 1000 Å thick with about 50 Å beingpreferred. Typical materials for barrier layers 20 may be semiconductorssuch as aluminum gallium nitride (Al_(x)Gal_(1-x)N), silicon germanium(Si_(x)Ge_(1-x)), aluminum gallium arsenide (Al_(x)Ga_(1-x)As) etc., orinsulators such as SiO₂ or Si₃N₄. The typical semiconductor materialsfor barrier layers 20 may be formed out of group IV (Si, Ge), groupIV-IV (SiC), group III-V and group II-VI. The smaller bandgap quantumwell layers 18 of quantum well structure 12 are preferably about 5 Å to1000 Å thick with about 5 Å to 30 Å being preferred. The gage factor ofpiezoresistor 10 increases as the thickness of the quantum well layers18 decrease. As the thickness of the quantum well layers 18 goes belowabout 30 Å, the gage factor of piezoresistor 10 increases dramatically.Typical materials for quantum well layers 18 are semiconductors such asgallium nitride (GaN), gallium arsenide (GaAs), indium phosphide (InP),silicon germanium (Si_(x)Ge_(1-x)), etc. The typical materials forquantum well layers 18 may be formed out of group IV (where the group IVelement may be Si,Ge,Si_(x)Ge_(1-x),SiC), group III-V (Solid solutionsof Al,Ga,In,N,P,As and Sb) and group II-VI (solid solutions of Zn,Cd,Hg,O,S,Se and Te). Each quantum well layer 18 is preferably sandwichedbetween two barrier layers 20 such that the top and bottom layers of thequantum well structure 12 are barrier layers 20. Alternatively thebottom barrier layer 20 may be omitted if an insulating layer 16 isemployed. In such a case, the insulating layer 16 forms the bottombarrier layer. The barrier layers 20 may be different compositions.

[0040] Typical methods of depositing layers 16, 18 and 20 over basesubstrate 14 are by physical vapor deposition (molecular beam epitaxy,sputtering, evaporation, etc.) or chemical vapor deposition. Suchmethods offer a very high degree of control for growth rate, backgroundimpurity levels and provide near atomic resolution which enables theformation of highly reproducible piezoresistors 10. The gage factor ofpiezoresistor 10 can be varied by varying the thickness of the layers18/20, and varying the doping type and/or density. The highest gagefactors can be achieved by depositing quantum well layers 18 being asthin as possible (about 5 Å to 10 Å). In addition, the bandgaps of thequantum well and barrier layers 18/20 can be tuned or varied bycontrolling the composition of the material in the layers 18/20.Electrical contacts 28 on quantum well structure 12 in one embodimentare fabricated by depositing suitable metals, for example by sputteringor e-beam evaporation, such that contact to all the layers of thequantum well structure 12 is made (FIG. 6).

[0041] An example of a preferred material combination for piezoresistor10 is as follows: 6H-SiC or sapphire for base substrate 14, AlN forinsulating layer 16, Al_(x)Ga_(1-x)N for barrier layers 20 and GaN forquantum well layers 18. Nitride semiconductor materials are preferredfor piezoresistor 10 due to stability at high temperatures (1200°) andin corrosive or reactive gas environments. Less expensive materials suchas column IV and lower gap III-V semiconductors, for example, Si, SiGe,Al_(x)Ga_(1-x)As, InP and GaAs can be employed for use at lowertemperatures or less corrosive or reactive gas environments. Anotherexample is alternating layers of amorphous SiO₂ (barrier) andpolycrystalline silicon (quantum well) deposited on a glass substrate.Alternatively, less costly semiconductor materials can be encapsulatedby more costly high stability materials such as nitride semiconductormaterials to reduce cost while improving the stability of the resultingpiezoresistor 10.

[0042] Using semiconductor processing techniques, multiplepiezoresistors 10 can be deposited on the same base substrate 14. Someof such piezoresistors 10 can be fabricated to have different gagefactors. Using masking and/or etching techniques, different types ofpatterns can be formed on base substrate 14 such as a trace or combstructures.

[0043] While this invention has been particularly shown and describedwith references to preferred embodiments thereof it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

[0044] For example, although a single crystal base substrate ispreferred, alternatively, a portion of the base substrate may be singlecrystal with other portions being polycrystalline or amorphous. In sucha case, a layer of single crystal material may be bonded to apolycrystalline or amorphous material. The quantum well structure isthen formed on the single crystal layer.

What is claimed is:
 1. A piezoresistor comprising: a base substrate; and a quantum well structure formed in conjunction with the base substrate, the quantum well structure comprising at least one smaller bandgap layer bounded by larger bandgap layers, the at least one smaller bandgap layer having a thickness of 30 Å or less.
 2. The piezoresistor of claim 1 in which the at least one smaller bandgap is a quantum well layer and the larger bandgap layers are barrier layers.
 3. The piezoresistor of claim 1 in which each smaller bandgap layer is 5 Å to 30 Å thick.
 4. The piezoresistor of claim 1 in which the larger bandgap layers in the quantum well structure are more than 5 Å thick.
 5. The piezoresistor of claim 1 in which the larger bandgap layers are 5 Å to 50 Å thick.
 6. The piezoresistor of claim 1 in which quantum confinement of carriers is achieved in the at least one smaller bandgap layer.
 7. The piezoresistor of claim 2 in which the quantum well structure has 5 to 10 quantum well layers.
 8. The piezoresistor of claim 1 in which the base substrate is a single crystal.
 9. The piezoresistor of claim 1 in which the base substrate is a mechanical element of a sensor.
 10. The piezoresistor of claim 9 in which the mechanical element is micromachined.
 11. The piezoresistor of claim 1 in which the base substrate is a thin film.
 12. The piezoresistor of claim 2 in which the barrier layers are formed from an insulating material.
 13. A piezoresistor comprising: a single crystal base substrate; and a quantum well structure formed in conjunction with the base substrate, the quantum well structure having alternating larger and smaller bandgap semiconductor layers, wherein quantum confinement of carriers is achieved in the smaller bangap layers, the smaller bandgap layers having a thickness of 30 Å or less.
 14. A method of forming a piezoresistor comprising: providing a base substrate; and forming a quantum well structure in conjunction with the base substrate, the quantum well structure comprising at least one smaller bandgap layer bounded by larger bandgap layers, the at least one smaller bandgap layer having a thickness of 30 Å or less.
 15. The method of claim 14 further comprising forming at least one quantum well layer from the smaller bandgap layer and barrier layers from the larger bandgap layers.
 16. The method of claim 14 further comprising forming the at least one smaller bandgap layer with a thickness of 5 Å to 30 Å.
 17. The method of claim 14 further comprising forming the larger bandgap layers more than 5 Å thick.
 18. The method of claim 17 further comprising forming the larger bandgap layers 5 Å to 50 Å thick.
 19. The method of claim 15 further comprising forming 5 to 10 quantum well layers in the quantum well structure.
 20. The method of claim 14 further comprising forming the base substrate from a single crystal.
 21. The method of claim 14 further comprising forming the base substrate from a mechanical element of a sensor.
 22. The method of claim 21 further comprising micromachining the mechanical element.
 23. The method of claim 14 further comprising forming the base substrate from a film.
 24. A method of forming a piezoresistor comprising: providing a single crystal base substrate; and forming a quantum well structure in conjunction with the base substrate, the quantum well structure having alternating larger and smaller bandgap semiconductor layers, wherein quantum confinement of carriers is achieved in the smaller band gap layers, the smaller bandgap layers having a thickness of 30 Å or less. 